1. Field of the Invention
The present invention relates to a LAN (local area network) employing a CSMA/CD (carrier sense multiple access with collision detection) system. In particular, the present invention relates to a method of, and an apparatus for, transmitting a large amount of data with priority in a LAN employing the CSMA/CD system.
2. Description of the Related Art
The CSMA/CD system has been developed as an access control system for Ethernet, which is a kind of a LAN, and has been standardized as IEEE (Institute of Electrical and Electronics Engineers) 802.3. The CSMA/CD system is widely used for bus-topology LANs and tree-topology LANs.
FIG. 1 shows relationships between IEEE 802.3 layers and OSI (open system interconnection) reference model layers.
IEEE 802.3 specifies a PLS (physical layer signaling) sublayer, an MAC (media access control) sublayer, and an LLC (logical link control) sublayer. The PLS sublayer corresponds to a physical layer of an OSI reference model, and the MAC and LLC sublayers correspond to a data link layer of the OSI reference model. The present invention relates to the data link layer and a network layer of the OSI reference model depicted with thick continuous lines in FIG. 1.
FIGS. 2 and 3 briefly explain the operation of the CSMA/CD system.
FIG. 2 shows collision detection by the CSMA/CD system, and FIG. 3 shows a resending operation by the CSMA/CD system after detecting a collision.
In FIG. 2, a transmission line 10 may be a twisted-pair cable or a coaxial cable. Nodes A to D (11 to 15) are connected to the line 10 and always monitor, in the PLS sublayer, a carrier detection signal that corresponds to a carrier signal transmitted through the line 10.
One of the nodes 11 to 15 transmits data if the node detects according to the carrier detection signal that none of the other nodes is using the-line 10. If any one of the nodes is using the line 10, the other nodes postpone transmitting data. A node starts to transmit data after a carrier detection signal disappears and after a minimum frame-to-frame spacing time (9.6 xcexcs) elapses.
If the node 12 transmits data to the node 15, and at the same time, if the node 13 sends data to the node 14, a collision occurs. Then, a jam signal or an interference signal is sent for a predetermined time (3.2 to 4.8 xcexcs) to all nodes (the nodes 11 to 15 in this example) connected to the same segment of the line 10 or contained in the same broadcast range to inform the nodes of the collision.
In FIG. 3, the nodes 12 and 13 that have caused the collision calculate respective back-off times according to a probability algorithm to avoid another collision. After the back-off times, the nodes 12 and 13 resend the data. In FIG. 3, the node 13 has a shorter back-off time, and therefore, succeeds to send the data to the node 14. At this time, the node 12 having a longer back-off time delays resending the data according to a carrier detection signal.
In this way, the CSMA/CD system gives the same priority to all nodes 11 to 15 connected to the line 10. Namely, the CSMA/CD system is unable to give priority to a specific node to access the line 10.
As the number of nodes connected to the line 10 increases, the traffic of the line 10 increases to rapidly increase the number of collisions. This deteriorates the transmission efficiency of the line 10 because many resending operations must be carried out as explained with reference to FIG. 3.
To solve this problem, Japanese Unexamined Patent Publication No. 05-300153 gives priority to a node and makes the node send a jam signal to forcibly stop communication between the other nodes. Thereafter, the node with priority starts transmitting data before the shortest back-off time elapses. To achieve the forcible stoppage of communication, this prior art uses the standard collision detecting and data resending functions in the PLS sublayer (corresponding to the physical layer of the OSI reference model).
An object of the present invention is to provide a method of, and an apparatus for, realizing priority data transmission for the CSMA/CD system. The present invention uses the data link or network layer of the OSI reference model depicted with thick continuous lines in FIG. 1. The present invention gives priority to a node to access a transmission line and makes the node send a frame or packet with a priority transmission notice through the data link or network layer to all nodes connected to the same segment of the transmission line to which the node in question is connected. Thereafter, the node in question temporarily exclusively uses the line to transmit a large amount of data in a short time without forcibly cutting communication between the other nodes.
In order to accomplish the object, the present invention provides a method of realizing priority data transmission for the CSMA/CD system, including the steps of checking to see if a transmission line has a free space in response to a data transmission request from a node serving as a source node, transmitting, if the transmission line has a free space, a priority transmission notice with a protocol signal from the source node to the other nodes in a data link layer of the CSMA/CD system, transmitting data for which the data transmission request has been made from the source node to a node serving as a destination node, and putting the nodes except the source node in a transmission-disabled, reception-enabled state when the priority transmission notice is received by the nodes.
The method may include the steps of transmitting a release notice from the source node, and restoring the transmission disabled nodes to a transmission-reception-enabled state when the release notice is received by the nodes or when a predetermined time elapses after the priority transmission notice is received by the nodes.
The method may include the steps of attaching a priority assigned to the source node to the priority transmission notice and transmitting the priority transmission notice from the source node to the other nodes, and comparing, at each of the nodes except the source node, the priority assigned to the source node with priority assigned to the node in question, and if the priority assigned to the source node is higher than that assigned to the node in question, putting the node in question in the transmission-disabled, reception-enabled state.
The protocol signal in the data link layer of the CSMA/CD system may be an MAC frame. The MAC frame has a frame type field and an LLC header field, one of the fields being used to carry the priority transmission notice with or without a value indicating the priority assigned to the source node, or the release notice.
Instead of the protocol signal in the data link layer of the CSMA/CD system, a protocol signal in a network layer of the OSI reference model may be used. The protocol signal in the network layer may be an IP datagram signal having an option field and a TOS field. The option field is used to carry any one of the priority transmission notice and the release notice. The TOS field is used to carry a value indicating the priority assigned to the source node.